Conventionally, there is a well-known hydraulic circuit including a hydraulic cylinder, a hydraulic pump driven by a drive source so as to deliver fluid to the hydraulic cylinder, and a motion switchover valve disposed between the hydraulic cylinder and the hydraulic pump so as to switch a motion of the hydraulic cylinder.
In the typical hydraulic circuit, generally, the motion switchover valve determines a direction of fluid delivered from the hydraulic pump so as to selectively supply the fluid to either a bottom chamber of the hydraulic cylinder or a rod chamber of the hydraulic cylinder, thereby extending or contracting the hydraulic cylinder. In this hydraulic circuit, when one of the bottom or rod chambers of the hydraulic cylinder is supplied with fluid, fluid is drained from the other of the bottom and rod chambers and is returned to a fluid tank through a return pipe. The hydraulic pump sucks fluid from the fluid tank.
Further, conventionally, as disclosed in JP 2000-257712A, a kind of the hydraulic circuit has a function referred to as “load sensing function.” The load sensing function is defined as a function to adjust a differential pressure of the motion switchover valve between a delivery port of the hydraulic pump and a suction port of the hydraulic actuator into a predetermined range so as to keep a substantially constant flow quantity of fluid supplied to the hydraulic actuator (i.e., to keep a substantially constant motion speed of the hydraulic actuator) regardless of variation of load applied on the hydraulic actuator.
Further, conventionally, as disclosed in JP 2001-207482A, a hydraulic circuit includes a hydraulic motor, a generator and a battery, so as to have a function referred to as “energy recovery function”. The hydraulic motor is disposed on an intermediate portion of the return pipe so as to be driven by fluid flowing in the return pipe. The generator is driven by the hydraulic motor and is connected to the battery. The energy recovery function is defined as a function to recover electric energy into which energy (kinetic energy and potential energy) of fluid flowing in the return pipe is converted.